“The real problem is Congress. They don’t really care about actually doing anything in space,” said Rand Simberg, an adjunct scholar at the Competitive Enterprise Institute and a leading space-policy journalist.

Internal NASA studies suggest cheaper and faster alternatives to the Space Launch System should be pursued. But work on the system is going forward because, Mr. Simberg said, it is a “politically expedient” option for members of Congress from states that are home to factories capable of building such rockets.